Gonadotropins serve important functions in a variety of bodily functions including metabolism, temperature regulation and the reproductive process. Gonadotropins act on specific gonadal cell types to initiate ovarian and testicular differentiation and steroidogenesis. The gonadotropin FSH (follicle stimulating hormone) is released from the anterior pituitary under the influence of gonadotropin-releasing hormone and estrogens, and from the placenta during pregnancy. FSH is a heterodimeric glycoprotein hormone that shares structural similarities with luteinizing hormone (LH) and thyroid stimulating hormone (TSH), both of which are also produced in the pituitary gland, and chorionic gonadotropin (CG), which is produced in the placenta. In the female, FSH plays a pivotal role in the stimulation of follicle development and maturation and in addition, it is the major hormone regulating secretion of estrogens, whereas LH induces ovulation. In the male, FSH is responsible for the integrity of the seminiferous tubules and acts on Sertoli cells to support gametogenesis.
The hormones are relatively large (28-38 kDa) and are composed of a common α-subunit non-covalently bound to a distinct n-subunit that confers receptor binding specificity. The cellular receptor for these hormones is expressed on testicular Sertoli cells and ovarian granulosa cells. The FSH receptor is known to be members of the G protein-coupled class of membrane-bound receptors, which when activated stimulate an increase in the activity of adenylyl cyclase. This results in an increase in the level of the intracellular second messenger adenosine 3′,5′-monophosphate (cAMP), which in turn causes increased steroid synthesis and secretion. Hydropathicity plots of the amino acid sequences of these receptors reveal three general domains: a hydrophilic amino-terminal region, considered to be the amino-terminal extracellular domain; seven hydrophobic segments of membrane-spanning length, considered to be the transmembrane domain; and a carboxy-terminal region that contains potential phosphorylation sites (serine, threonine, and tyrosine residues), considered to be the carboxy-terminal intracellular or cytoplasmic domain. The glycoprotein hormone receptor family is distinguished from other G protein-coupled receptors, such as the β-2-adrenergic, rhodopsin, and substance K receptors, by the large size of the hydrophilic amino-terminal domain, which is involved in hormone binding.
Annually in the U.S. there are 2.4 million couples experiencing infertility that are potential candidates for treatment. FSH, either extracted from urine or produced by recombinant DNA technology, is a parenterally-administered protein product used by specialists for ovulation induction and for controlled ovarial hyperstimulation. Whereas ovulation induction is directed at achieving a single follicle to ovulate, controlled ovarial hyperstimulation is directed at harvesting multiple oocytes for use in various in-vitro assisted reproductive technologies, e.g. in-vitro fertilization (IVF). FSH is also used clinically to treat male hypogonadism and male infertility, e.g. some types of failure of spermatogenesis.
FSHR is a highly specific target in the ovarian follicle growth process and is exclusively expressed in the ovary. However, the use of FSH is limited by its high cost, lack of oral dosing, and need of extensive monitoring by specialist physicians. Hence, identification of a non-peptidic small molecule substitute for FSH that could potentially be developed for oral administration is desirable. Low molecular weight FSH mimetics with agonistic properties are disclosed in the international applications WO 2002/09706 and WO 2010/136438 as well as the U.S. Pat. No. 6,653,338. There is still a need for low molecular weight hormone mimetics that selectively activate FSHR.